This invention relates to a method of forming crystalline silicon films. Various methods are known for forming crystalline silicon films including methods for the crystallization of previously formed amorphous silicon films, and methods for the direct deposition of crystalline silicon. For example, the following methods are known:
(1) After an amorphous silicon film is deposited on a substrate by an electron beam vapor deposition process, it is irradiated with laser or electron beams to convert it into a crystalline silicon film. PA1 (2) When an amorphous silicon film is formed on a substrate heated to 1000.degree. C. or higher by a chemical vapor deposition (CVD) process, the amorphous silicon film is crystallized and a crystalline silicon film is formed. PA1 (3) An amorphous silicon film is heat-treated at 700.degree.-1000.degree. C. to convert it into a crystalline silicon film. PA1 (4) A crystalline silicon film is formed by a plasma CVD process using a reaction gas of monosilane (SiH.sub.4) gas diluted by hydrogen to its concentration of 5% or less. PA1 (5) A crystalline silicon film is formed by a plasma CVD process using a reaction gas of a mixture of monosilane with helium gas.
Each of these known methods, however, suffers from disadvantages due to complex and expensive equipment or to extreme reaction conditions.
For example, deposition of amorphous silicon on a substrate using electron beam vapor deposition, followed by irradiation with a laser or an electron beam to crystallize the silicon not only involves elaborate apparatus, it also results in imposing thermal strain on the material due to the high temperatures generated during the crystallization process. High temperature is also a problem when amorphous silicon films are deposited by chemical vapor deposition processes and then recrystallized. Such processes are frequently carried out at temperatures in excess of 1000.degree. C. Related processes in which crystallization results from simply heating amorphous silicon to 700.degree.-1000.degree. C. also cause thermal stress.
Crystalline silicon films can be deposited directly using a plasma CVD process. In these processes the mixture of silane and carrier gases is critical, and must be controlled relative to the glow discharge power. Moreover, both processes have slow deposition rates and poor uniformity of the crystalline film because of concentration of the discharge power density near the middle of the electrode.
Besides these drawbacks, none of these processes, with the exception of laser or electron beam irradiation, is conducive to selective crystallization of a part of an amorphous silicon film. Such selective crystallization is highly desirable in the fabrication of a number of different device types.
It is therefore an object of this invention to provide a method for forming crystalline silicon films at relatively low temperatures.
It is a further object of this invention to provide a method for crystallizing defined areas in an amorphous silicon film at relatively low temperatures.